The present invention relates to soft switching high voltage and resonant power converters. It is well known in the art that DC transmission is more efficient than AC transmission. Additionally, the construction of AC transmission lines is more costly per mile than an over-land High Voltage Direct Current (HVDC) lines. Conventionally, most power is generated at approximately 11 kV AC and is consumed by end users as AC power. In order to transmit generated AC power as HVDC power, the AC power is first converted to HVDC power. After transmission over HVDC lines to an end user, the HVDC power is converted back to lower voltage AC power. These conversion processes have large losses and are costly. Thus, the conversion process is generally only economical when transmitting high power, high voltage DC over long land distances using “classical” line commutation converter (LCC) technologies.
When transmitting power over distances greater than 25 miles using marine cables, the reactive AC current in the cable forces the use of DC transmission. The use of such lower power DC cables is becoming prevalent as more wind power generators are installed off-shore.
One conventional type of power converter used for lower power DC transmission is the PWM voltage source converter (VSC). PWM voltage source converters have higher losses than “classical” line commutation converters, but are often more cost effective for lower power levels.
The major losses of high voltage PWM voltage source converters generally arise from turn-on and turn-off switching losses due to the “hard-switched” (HS) topologies of the converters. The switching losses drastically increase as the voltage rating of the converter switches increases. The switching losses also increase proportionally with the switching frequency of the converter. An increased switching frequency not only adds to the system losses, but also prevents the converter designer from using higher voltage (6.5 kV and potentially higher in the future) solid-state devices, since for a minimum required switching frequency, a junction temperature of a conventional switching device will exceed a safe operating temperature. The limitation imposed by high converter switching frequencies forces the use of a number of lower voltage devices in series to yield the required voltage hold-off requirement. Such an increase in the number of devices used in the converter design increases the size, cost, complexity, conduction losses, and system failure rate of the converter.
The DC to AC VSC revolutionized the variable speed control of industrial motors in the voltage range below 600 V. This resulted in the advancement of IGBTs as a work horse switch. However, the uses of this converter topology were found problematic for high voltage applications due to several well known switch limitations, dV/dt, standing voltage waves, and other reasons. In addition, the poor efficiency of VSC converters due to hard switching losses makes VSCs unattractive for efficient utility power conversion applications. To use VSCs for higher voltage traction motor applications a number of advanced multi-level VSCs have been devised. For example, U.S. Pat. No. 6,005,788 to Lipo proposed a multi-level DC to AC converter using multiple DC power sources and a number of series connected hard switched converters in a hybrid PWM configuration “connected set of H-bridge inverters”.
The elimination of the multiple DC power sources has been advanced by the various “Diode Clamped Multilevel Inverters”, such as those published by Xiaoming Yuan. In general, Diode Claimed Multilevel Inverters operate as DC to AC converters in a PWM mode.
A multilevel converter for a single AC input phase AC to DC railroad traction application is described by Lue Meyses in U.S. Pat. No. 7,558,087 B2. The multilevel converter includes a number of transformers for galvanic isolation using a hard switched topology limited to a switching frequency “below 3 kHz”.
Finally, many conventional systems are constructed with a number of switches directly connected in series to yield the necessary full voltage hold-off requirement. The switches need to be accurately turned-on and turned-off simultaneously to dynamically share the voltage of the stacked switch assembly. This requirement on switch timing is a large system risk and causes a majority of system failures.